Wednesday, 26 June 2019

Quick Field Strength Meter using el-cheapo multimeter

This quick and dirty field strength meter ive knocked up really isnt doing well! It would just about deflect with a 3W VHF handheld right by its antenna. So, I added a FET...

Very simple, its now little more than a crystal set with a 2N3819 FET and a 5k potentiometer. Powered from 3v, the pot allows me to swing the needle across the whole deflection range, yet when held within inches of a 4W lowband VHF transmitter - barely a movement!

Oh well, i'll look at it more tomorrow. Maybe its because im only using 3v instead of the customary 9v? Or, maybe its the meter movement isnt sensitive enough? But the multimeters minimum DC current range was 500uA, so im presuming its a 500uA movement?. Or perhaps its the fact that the original 'dc path' resistor, which is now the FET gate bias resistor, is just 10k...

Tomorrow, i'll swap that resistor for a 1M unit, and see how that works.

Oh well, couldnt wait. Went and added a 1M in series with the 10k, measured the meters FSD current (its near enough 0.5mA to call it a 500uA movement), even tried the FET the other way around (2N3819s are near enough reversible), and it still doesnt work! Oddly, close to the 4W Tx the movement deflection is actually towards the lower end of the scale!

Only thing left to try is the battery...

Why Bloody PP3s?

So many designs, projects etc, are powered by the poxy 9v PP3 battery! Why? They have sod all capacity, and cost a fortune! Yes, I know they are conveniently sized and shaped, but half the things I see powered by them could just as well be designed to run on a pair of AA's at little more space and a fraction of the cost.

Earlier today, simply because I came across the detector innards from a previous version in a box, I knocked up a Field Strength Meter, using the meter movement from a defunct chinese cheapo multimeter, and the first 3D printed box Alex at work made for me. A spare telescopic antenna fitted, and its a conveniently handy sized device. But, its rather insensitive due to the meter movement I used, so really needs an amplifier. Hence the above rant - all the designs use a PP3! Im looking for a design now that uses 3v so I can power it from a couple of AA's,

The remaining seven Low Battery Warning modules are now built, and offered for sale. One initially failed testing, this turned out to be the collector pin of the optocoupler that had failed to solder properly. All of them switch to alarm state between about 19.9-20.5v. Not bad for the component tolerances.

Ive offered these for sale on the Clansman Field facebook page, at £10 each. For every one sold, im putting £4 into the coffers for our charity fundraising!

Friday, 21 June 2019

More on the Static Bleed Unit

Returning to this project this morning, having had the night to dream up what exactly I wanted it to do, I decided that it had to do the following -

1. Allow for both ferruled and bare earth wires
2. Allow for bare antenna wire, ferruled wire, coax and 4mm banana plug antenna connections
3. Allow for coax, 4mm banana and ferruled wire connections to radio

I have managed to accomplish all but the ferruled radio connection. I will add this later, but for now ive run out of springy D10 terminals!

Internally, a bank of parallel 2W 1.5Mohm resistors form a high power 250k static bleed to ground. 250k is high enough to not affect tuning even on the most temperamental end fed wires, but low enough to quickly drain any static build up. Three extra antenna connectors have been added, so along with the Clansman stud connector, and the BNC socket, there is a 4mm banana plug socket, a screw type insulation piercing terminal and a sprung terminal. For the radio, there are options to connect via coax to the BNC, or 4mm plug. A further sprung D10 terminal will be added once I have some.

View from the side of the antenna terminals.  The BNC antenna socket can also be seen.

Radio end! The SURFs own little coax patch lead and holder remain, but a new 4mm socket allows for single wire connection. This socket was salvaged from an old IF injector and was a bit of a pig to install! 

View from the grounding side. The srpung terminal makes use of the hole forom the SURFs tuning control. Ive left the original meter cover in place, and found a rubber bung left over from something or other fits the hole left by removing the meter perfectly!

And thats it with the lid on! I just need to remove that god awful asset sticker!

Thursday, 20 June 2019

Repurposing a 4W SURF into a Static Bleed Unit

One of the problems with lifting antenna wires using kites or balloons, is that wind blowing across the wire generates static. This can build up to tens of thousands of volts - enough to destroy modern front end transistors. If you want to play with kite antennas, you have to remove this static.

To do this, is really simple. You simply make a box with some high power, high value resistors in it, with connections for the antenna wire, the feed to the radio, and a connection to earth. Most people would do this using a little die cast aluminium box.

I wanted a design that did two things, 1. Used military connectors and would look the part next to other military radio equipment, and 2. wouldnt eat into my small stock of now very expensive die cast boxes!

Something that is much cheaper than a die cast box, is a Clansman 4W SURF unit!

As I had one spare, I decided I would remove the electronics, and repurpose it! The electronics, which include a beautiful silver plated air spaced variable capacitor, have been retained for spares.

The empty case has quite a few mouldings that make deciding exactly how to repurpose it a bit tricky! It turns out that the dehydrator plug hole is far too deep to insert anything in, and the meter hole far too wide. The hole for the tuning control though, was only a little too deep - but accessible! Its the little stub of aluminium that can be seen on the top left of the picture at the left end of the central divider. Ten minutes work with a file had this stub removed!

As I wanted two ground terminals, I decided the other should go below and slightly to the side of this hole, and so drilled a new hole for it. Here I made a little mistake as I didnt notice that internally this point in the case is sloped! Plus, it meant the nut was underneath the stud for the lid fixing, but I managed to get it tightened up successfully.

So here it is so far. Two ground terminals installed. The upper terminal is a sprung push type D10 terminal, salvaged from a defunct CRS/LR box. I had a bit of a shock when I opened this box and found it wet inside! Odd for a sealed box! I believe it to be water, but there were a lot of odd white crytsals inside as well! The other terminal is a screw type insulation piercing D10 terminal taken from a defunct PTC404 field telephone.

The electronics removed from that PTC404 are shown above. The build quality of these field telephones is absolutely superb!

The CRS/LR box and the PTC404 were part of a batch of kit I picked up today from Dominic at LRseries surplus. This batch included the antenna elements, base, and cables I need to get the TUAAM working, plus an EVHF antenna to play with, and a few spare audio plugs, to use to make up test leads.

Another item in the batch was a damaged battery extension cable. Much of the cable itself had failed, seemingly due to fuel or oil exposure. I found about 2ft of it that was usable, cut it out, and set to making it into something useful

The connector ends turned out to be filled with soft silicone rubber. This had to be picked out (easy enough) before I could access the terminals.

With the good cable reinstalled into the radio end connector, A couple of spare 4mm plugs later and I now have a very useful external power connector. I can now easily connect radios up tot he bench variable PSU for testing.

Despite everything, once I was home from LRseries, I thought of loads of other stuff I should have got! I also think I might want a couple more D10 terminals than I have! So looks like i'll have to go back again in a week or two!

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Wednesday, 12 June 2019

Inside the Racal BCC425 Initiate Box

Just in case anyone was wondering what was in the Clansman Initiate Box, well heres a photo

As you can see, its really very simple inside. Probably the easiest bit of Clansman to repair!

Support our charity fundraising! -

Did I mention our charity walk?

Those of you who frequent this blog may also be aware of my other, seldom updated walking blog. Well, myself and Bob M1BBV are yet again taking on a rediculously long walk for charity!

Next month, we will walk 84 miles in just 3 days, to conquer the Hadrians Wall long distance path. This is to raise funds for Cancer Research UK.

If you like my blog, and if anything ive put in my ramblings has been helpful to you, why not consider saying thanks with a bit of sponsorship?

You can donate to our cause at our dedicated fundraising page here

Please help us hit our target and help find a cure for cancer!

I'll be adding the link to each blog post as well.

Monday, 10 June 2019

Up, Up and Away!

Ive finally got around to starting the work on fitting the ADS-B receiver system into a weatherproof housing for mast installation. I happened to obtain an ex-equipment outdoor unit housing, and realised I could repurpose it from its original job of line protection unit.

Ive used the same antenna, but with some blind grommets and silicone. Mounting the electronics took some thinking about to ensure good all round ventilation and airflow, and access to the SD card.

I need to get some better blanking plugs for the holes before it can go outdoors. and work out how im going to attach mast mounting brackets.

Ive also done the necessary repairs to the rescue box kite. This involved learning how to completely dismantle it in order to take off the wing fabric in order to attach the repair tape, and to fix the broken longeron.

Im now waiting for the tapes adhesive to cure and then can fold it back up again.

PRC344 low battery warning v.1.0 - 1st working

After a lot of fiddly work with these tiny 0603 sized SMT parts, and the retrofit of a standard 1/4w resistor to effect the required virtual ground, I now have the very first of the version 1.0 boards installed and tested in my PRC344.

Fiddly work - those black specks are the resistors!

The chosen resistors for Ra and Rb actually, in use, give an alarm threshold of 20.86v. Thats a little higher than I was aiming for, but not bad for fixed value 5% components.

Completed board

Insulated inside heat-shrink ready to install

The radio is now back together, tested on the variable bench PSU, and now sat monitoring North-East ICF on 270.050MHz, where there is some activity. Rather annoyingly, there is a dreadful sproggie from my PC right on that very frequency! So I have to move the radio quite a way clear of the computer.

I now need to let the radio drain down the battery until the alarm starts, and then check the battery voltage and the alarm function.

Sunday, 9 June 2019

Project Delays

Its been some time since I posted on here, mostly due to being worked like a dog on overtime, but also some big household projects needed working on, and also training yet again for a charity walk.

Ive revisited the PRC344 battery warning project today. I'd forgotten about the modification needed to the v.1 PCBs! This is because I was let down by a supplier and am still waiting for the 100k and above resistors.

Needing to get at least some of these v.1 boards completed and up running, I tested the circuit on breadboard again tonight using 10k range resistors instead, just to prove that it works that way, which it does. I also tested various combinations of Ra and Rb in the 1-10k range and found a couple of combinations that give a 20.2v cut-off (4k7 and 6k8) or 18.7v cut-off (3k9 and 6k8). I'll have to check the minimum terminal voltages of the batteries and decide which to fit. I think though that 1v/cell really is best as a maximum discharge, so the 20.2v values will probably be chosen.

This will allow me to get at least a few of the boards working, at least until I run out of 0603 10k resistors! But I should at least be able to demonstrate one! Then, when the rest of the resistors arrive, the remaining boards can be made up with either 10k or 100k series.

Its now however almost certain that I will redesign for a v.2 board using 0805 size parts, this will allow the use of a better voltage rated and higher value capacitor for the timing, but I will also add the duty cycle control diodes as well.

Ive also, since last posting, acquired a WW2 rescue box kite (as used with the famous 'Gibson Girl' lifeboat transmitter) which I have repaired and restored. I am now in the process of building an antenna connection and protection unit, which will drain wind static and protect against electrical discharge. Once that is done, it will be time to attach an antenna wire, fly the kite and load up the Clansman!

Wednesday, 20 March 2019

Fatal Flaws!

Another nasty realisation looks like it might be the death of the first version of the PRC344 battery warning module!

Yesterday it came to light that a further 100k resistor was required. Not a difficult thing to add in itself. But while shopping for the 4M7 resistors needed for the timing, and realising that they are rather expensive, a consideration of the alternative, that is going to a 10uF capacitor and dropping the resistor down to 470k, led to the discovery that the 1uF ceramic capacitors im using are only rated at 16V!

I really dont know how far I could safely push that rating! A quick test shows that the voltage on the capacitor peaks at about 16V for a 24V supply. But, a freshly charged battery could easily be up to near 28V! And there are also single use 33V Lithium batteries out there! A check at 28V showed the capacitor to have over 19V on it!

There are 10uF 35V SMD tantalum capacitors available at good prices, but these are considerably bigger than the 1uF 0603 ceramic! No chance of a retro-fit mod here!

So, although I have a fair bit of cash tied up in the parts stock for these v.1 modules, it is looking risky to sell them as capable of being reliable in a 24V Clansman system. One option might be to add a potential divider on the supply line to keep the voltages safe of course! I will test that out. The whole module takes only a couple of mA so it should be a safe method, if a little untidy.

But another issue is that a non 50/50 duty cycle is preferred, and to do that requires extra parts anyway. I will test this out on breadboard, but whether I then invest in a v.2 module for sale will have to depend on if there is likely to be enough demand. I am also however going to test out whether or not I can do away with the switching transistor.

So the v.1 warning module looks like it might be a bit of a hybrid SMT/through hole unit, with fixed 3sec on/3sec off warning. This would at least save the project!